Tolerant integrated circuit to great manufacturing faults

Abstract: The integrated circuit tolerant of large manufacturing defects comprising a first plurality of first conductors (CO1) made of a first material with relatively low conductivity and each having a plurality of first electrical connection points (CP) arranged along itself and a second corresponding plurality of second conductors (CO2) made of a second material with relatively high conductivity and each having a plurality of second electrical connection points (CP) arranged along itself and said plurality of first points are electrically connected to said plurality of second points respectively in such a manner as to reduce the series resistance of the first conductors and the second conductors (CO2) are interrupted between some second consecutive points (CP) in such a manner as to leave relatively large areas of the integrated circuit not traversed by the second conductors (CO2).

Description: The present invention relates to memory device in accordance with the preamble of claim 1. Memory devices always comprise at least one cell matrix comprising a plurality of rows, a plurality of columns and, in correspondence with each crossing between the rows and columns, a more or less complex memory cell. The rows and columns consist generally of electrical conductors made of silicide or polysilicon which are materials with relatively low conductivity. Since memories have ever growing capacities and must meet stringent specifications as regards access time, many manufacturers have elected memory devices where the rows are provided conventionally while the columns are provided of metallic materials having relatively high conductivity and making them from a first metallic layer (metal-1). To make the devices still faster it has been necessary to reduce the series resistance of the rows. This was achieved by means of another plurality of electrical conductors running parallel to and superimposed on the conductors of the rows and contacted therewith by means of the opening of VIAs every certain number of memory cells as may be seen in FIG. 1. The conductors of said additional plurality are made of metallic materials having relatively high conductivity and making them from a second metallic layer (metal-2). The process for making from the second metallic layer the additional plurality of conductors is subject to even large defects for a variety of reasons one of the many of which is the difficulty of planarisation of the underlying structure. Said defects result in short circuiting of two or more adjacent conductors depending on the size of the defect as may be seen in FIG. 1. It is common practice to obviate said defects by providing redundant rows. In general redundancy is limited to short circuits between two adjacent rows for reasons of failure frequency and redundancy management circuitry simplicity and, for flash EPROM memories, the space occupied by the row and/or column decoding circuitry. Indeed, flash EPROM memory devices can be erased only in their entirety while they can be written one cell at a time. If a cell were never written but repeatedly erased (together with the others) it would be "emptied" in a short time and this would lead to read errors for the cells located in the same column. To obviate this shortcoming, before any erasing phase of the device there is a "preconditioning" phase affecting individually all the cells of the device (those of the perfectly operating rows, the defective ones and also the redundant ones) and consisting of writing the cells not already written by injecting charges in the insulated gate of the MOS transistor which makes them up. When a write operation is executed in a row exhibiting a short circuit it is necessary anyway that the cells making it up be actually written so as to avoid future read errors. To do this there are needed stronger and more cumbersome generators in proportion to the stronger current needed to pilot the row despite the short circuit and this depends on the number of short circuited rows and the size of the defect. Present integration levels do not allow dedicating to said generators a very big area on the chip. Consequently only short circuits between two adjacent rows are made redundant. The short circuit of multiple rows constitutes an obstacle to operation of the entire device and cannot be accepted nor managed by means of software or firmware avoiding the use of such defective rows (this is naturally true only for flash EPROM memory devices) and hence results in rejection of the device. A known prior art solution is disclosed in US Patent No. 4,827,449 which relates an integrated circuit of the type comprising: a first plurality of polycide first conductors 13 each having a plurality of first electrical connection points 16 arranged along itself, and a second corresponding plurality of aluminium second conductors 14 each having a plurality of second electrical connection points 14c along itself and said plurality of first points being electrically connected to said plurality of second points respectively in such a manner as to reduce the series resistance of the first conductors. However, in such a reference integrated circuit there isn't a redundant conductor arrangement taking care of larger defects or improving the fault coverage of short circuits of adjacent conductors. The purpose of the present invention is to provide a memory device having a structure such as to be tolerant of large manufacturing defects. Said purpose is achieved by means of the memory device of the above mentioned type having furthermore the characteristics set forth in the characterising part of claim 1. Further advantageous features of the present invention are set forth in the dependent claims. By having the second conductors interrupted between some second consecutive points in such a manner as to leave relatively large areas of the integrated circuit not traversed by the second conductors the probability that a defect would affect two conductors causing a short circuit between them is reduced and for three conductors it is reduced even further. The resulting increase in the series resistance is acceptable for memory devices. Furthermore, thanks to the use of redundant rows for said devices rejects due to manufacturing defects and in particular those linked to the second metallic layer are practically eliminated. The invention is clarified by the description set forth below considered together with the annexed drawings wherein: FIGS. 1, 2 and 4 show top views of second conductors in accordance with the known art, FIG. 5 shows a top view of first and second conductors in accordance with the present invention, and FIG. 3 shows a cross section view along plane of cut A-A of the conductors of FIG. 2. FIG. 1 shows a second plurality made up of six second metallic conductors C02 which exhibit various electrical connection points opposite positions CP. Said conductors could be word lines of a memory matrix. Beneath the second conductors C02 and along their entire length there is a corresponding first plurality made up of six first conductors CO1 (not shown in FIG. 1), e.g. silicide, exhibiting various electrical connection points opposite the positions CP. Again opposite the positions CP there are provided VIAs permitting electrical connection between the conductors CO1 and the conductors C02. FIG. 1 shows a defect D-O such as to short circuit two adjacent conductors CO2. FIG. 2 shows the second conductors CO2 of FIG. 1 as modified in accordance with US-A-4 827 449. As may be seen these are interrupted every two consecutive points CP and the breaks are in alternating positions. In the areas where the conductors CO2 are interrupted are visible the conductors CO1 which are however hidden by the conductors CO1 in the remaining areas. In this way there remain relatively large integrated circuit areas not traversed by conductors CO2. This means for example that the defect D-O no longer causes any short circuit between the conductors CO2. As mentioned above the positions of the breaks in the conductors CO2 are determined in such a manner as to leave relatively large areas of the integrated circuit not traversed by the second conductors CO2. These depend thus on the mutual positions of the conductors and the size of the defects which it is intended to tolerate. FIGS. 2 and 5 show two of the many possible choices. In the case of memory devices the positions CP are found in general at a constant distance every certain number of memory cells, e.g. 512. In other applications this might not be the optimal choice. In general the positions CP are determined on the basis of considerations on the electrical signal propagation time in the conductors CO1 and hence depend also on the type and number of electronic devices connected thereto. FIG. 3 shows a cross section view along plane of cut A-A of the conductors of FIG. 2 as if these belonged to a memory device. In the cross section of FIG. 3 are shown the first conductors CO1 which act as word lines of a memory device and are arranged above a substrate BU and separated therefrom by insulating layers IS. In the substrate BU are made pockets SO and DR which act as source and drain areas respectively for MOS devices constituting memory cells. Above said structure is a dielectric layer DL in which are arranged third metallic conductors C03 acting as bit lines of the memory device. Above the dielectric layer DL are second conductors CO2 opposite some of the first conductors CO1. The present invention concerns memory devices. With the aid of FIGS. 4 and 5 the invention is explained below. As mentioned at the beginning, it is common practice to provide in memory devices redundant rows to obviate short circuits between two adjacent rows due to manufacturing defects. FIG. 4 shows a plurality of rows of such a memory device and three defects D-1, D-2 and D-3 caused by the process for making the rows in a metallic layer. The defect D-2 results in a short circuit between two adjacent rows and hence could be recovered by making use of redundancy. The defects D-1 and D-3 would cause rejection of the device since they involve a short circuit between four and six adjacent rows respectively. Using the lesson of the present invention the rows of FIG. 4 were interrupted at several points giving rise to the rows of FIG. 5. In practice, pairs of rows were taken into consideration, the rows of each pair were interrupted at the same positions, and adjacent pairs were interrupted in alternating positions. As may be seen in FIG. 5 now the defect D-2 no longer causes problems because it does not affect any row and the defects D-1 and D-3 affect a short circuit of only two adjacent rows and hence can be redundant without causing rejects. In this manner there can be achieved a statistical reduction of rejects.

Claims: Memory device comprising a plurality of word lines each consisting of: a) a first conductor (CO1) of a first material with relatively low conductivity and having a plurality of first electrical connection points (CP) arranged along itself, and b) a second conductor (CO2) made of a second material with relatively high conductivity and having a plurality of second electrical connection points (CP) along itself corresponding in spacing to that of said first connection points, said plurality of first points being electrically connected to said plurality of second points respectively in such a manner as to reduce the series resistance of the first conductor, said second conductors being interrupted between alternate pairs of consecutive points ; characterised in that the word lines form a plurality of groups of neighbouring word lines, each comprising at least a pair of word lines, the second conductors within each group of word lines being interrupted between adjacent pairs of connection points so that the portions of uninterrupted second conductor an neighbouring word lines lie directly next to each other so as to be aligned completely over their length, the second conductors in adjacent groups of word lines being interrupted between alternate pairs of connection points so that the uninterrupted portions of second conductor in these adjacent groups are in staggered arrangement so as to have no overlap along their length. Memory device in accordance with claim 1 in which the distances between first and second consecutive points are equal. Memory device in accordance with claim 1 in which said first and second points are electrically connected together by means of vias. Memory device in accordance with claim 1 in which said first material is polysilicon or silicide and said second material is metallic. Memory device in accordance with claim 1 in which said first conductor and said second conductor are superimposed. Memory device in accordance with claim 1 in which the memory cells are of the flash EPROM type.

Inventors: MAZZALI STEFANO, MAZZALI, STEFANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L23
IPC codes: H01L23, H01L27